Sciencemadness Discussion Board

Fractional steam distillation under reduced pressure

Actinium - 15-10-2014 at 22:42

So I have been doing steam distillation for some time now and went back to vacuum distillation a few days ago and did well for myself. I then tried performing a fractional steam distillation under reduced pressure using my trusty aspirator, and needless to say it was a hot mess.
I've been practicing on essential oils as there are many fractions (Factional dist), Immiscible with water, (steam dist). Great to practice reduced pressure for those high B.P. fractions. Here where the problem occured.
The oil and water lower the B.P. form an Azeotrope and come over, the vacuum lowers the B.P. so as soon as the vaccum was applied to oil/water it was like a volcano and whooosh over it came...
any good advice how one would perform this specific distillation? Is it important to calculate all compounds prior or just go for it slow and low?

DJF90 - 16-10-2014 at 00:10

Always place your mixture under reduced presssure, then apply heat. Steam distillation with an external steam source will probably not work (volcano as you suggested...). Not that you should need reduced pressure, as that eliminates the requirement for steam.

Water does not form an azeotrope with the oil; steam distillation relies on the volatile components (essential oils) having a partial pressure at the steam temperature according to Raoult's law, and they pass over with water as a vapor phase, separating on condensation due to immiscibility. A higher proportion of oil will pass over if you use superheated steam (pass the steam through a copper spiral heated over a bunsen before the inlet tube) as the oils will have a higher partial pressure.

Conversely, I can imagine that under reduced pressure the lower steam temperature means lower partial pressure of oil and thus a larger amount of steam will be required for the same recovery.

Magpie - 16-10-2014 at 17:59

If a substance is imiscible in water and does not react with it, it can be steam distilled. The distillation does not follow Raoult's Law but instead each substance supplies its total vapor pressure, ie, the pure substance vapor pressures are additive. When the sum = 760mmHg (1 atm) the pair will codistill. Also the bp will be lower than that of either pure substance. That is why I do not see why one would use vacuum distillation while steam distilling, as the bp would be below that of water. A bp below 100°C is normally not a problem for oils.

For example 1-octanol has a bp of 195°C and is imiscible with water. It steam distills at 99.4°C. When steam distilling the bp is always going to be less than 100°C.

DJF90 - 16-10-2014 at 23:15

Sorry Magpie. Turns out to be Dalton's law, not Raoult's. Vogel discusses Steam distillation in relative detail in during one of the first few chapters.

Actinium - 17-10-2014 at 07:20

At magpie-
this is where I was having trouble understanding. Does the total sum of both compounds pressure need to be equal to 760mmHg (1 atm) to be steam distilled? or only when it equals that do you figure out its B.P.? I recall seing a formula expressed as, P = P(a) + P(b) is this which you are talking about?
If that is the case then do you figure the total pressure of both liquids by combining the pressure of both.. the question is when a compound has multiple compounds/fractions with various B.P. and pressures where do you begin?

At DJF90-
My reasoning behind the reduced pressure was to speed up the process which the distillation occured.
Also you are correct its Dalton's law of partial pressure..
Chemwiki had this to say about Raoult's Law.
"he sums of the values predicted by Raoult's law for the two liquids individually, but in general, this does not happen. The reason for this can be understood if you recall that Raoult's law reflects a single effect: the smaller proportion of vaporizable molecules (and thus their reduced escaping tendency) when the liquid is diluted by otherwise "inert" (non-volatile) substance."

Magpie - 17-10-2014 at 08:51

Quote: Originally posted by Actinium  
At magpie-
this is where I was having trouble understanding. Does the total sum of both compounds pressure need to be equal to 760mmHg (1 atm) to be steam distilled? or only when it equals that do you figure out its B.P.? I recall seing a formula expressed as, P = P(a) + P(b) is this which you are talking about?


The vapor pressure of a substance increases as its temperature increases. When the temperature reaches a point where the sum of the pure substance vapor pressure of water (Pw) and that of the substance (Ps) equals atmospheric pressure (Pw +Ps = 1 atm) the two will codistill. This temperature will necessarily be below the normal boiling point of both the water and the substance.
Quote: Originally posted by Actinium  

If that is the case then do you figure the total pressure of both liquids by combining the pressure of both.


Yes.
Quote: Originally posted by Actinium  

... the question is when a compound has multiple compounds/fractions with various B.P. and pressures where do you begin?


I suspect that this situation could be rather complex. My guess is that the multiple oils, being mutually soluble, will present a vapor pressure and composition in line with Raoult's Law. This mix will then codistill with the immiscible water as usual.






[Edited on 17-10-2014 by Magpie]

Actinium - 20-10-2014 at 00:06

thank you for clarifying these things.
Any good excercises you could suggest as to practice both the distillation as well as the math/calculation part of it? learning how to predict the outcome?
Its important to me to learn the on paper aspect of Chemistry as it is to learn the practical/ hands on lab work that we all want to do.
Seriously who doesn't want to be a wizard.;)

Magpie - 20-10-2014 at 16:17

Pavia et al in their lab manual Organic Laboratory Techniques, provide a good write-up on steam distillation. And yes, there is much that can be predicted mathematically based on the vapor pressure-temperature data and molecular wt of the oil. You can not only predict the bp but also the mole fraction of the oil in the oil-water distillate.

Pavia has an experiment in steam distillation. He suggests using one of the following spices: allspice, cloves, cumin, caraway, or fennel. I chose the cloves. Pure clove oil smells very nice. :)

Metacelsus - 20-10-2014 at 16:21

I did steam distillation of cloves in an organic chemistry class. Clove oil does indeed smell great at first, but the smell gets tiresome after a while.

Actinium - 20-10-2014 at 23:47

I had a bottle of cloves on my spice rack and decided to distille it. stuff loves to linger in the glassware
Thanks magpie. I'll have to check that out.